The effects of DF and protein were studied in solid (vegetable patties, gel), semisolid (porridge) and liquid (beverage) model foods. Depending on the study, psyllium, oat and wheat brans and enzymatically depolymerized oat bran were used as DF source, and soy protein, caseinate, enzymatically crosslinked caseinate and whey protein as protein sources.
5.1.1 Effects of dietary fibre and its structure modification (Studies I–III) Gastrointestinal hormone responses
Fibre enrichment and fibre type. Dietary fibre enrichment (10.0% DF, psyllium) of vegetable patties strongly modified postprandial hormonal responses. Psyllium enrichment of solid vegetable patties attenuated the typical decline of postprandial ghrelin concentrations and lowered the total ghrelin response (AUC) compared with the low-fibre low-protein product (p<0.05, Study I) (Figure 7 C). Psyllium enrichment modified also the postprandial GLP-1 response (Study I) (Figure 7 D). The psyllium-rich high-protein product suppressed the GLP-1 response for the entire experimental period, and the response was significantly lower compared with the low-fibre products (p<0.05) (Figure 7 D). Psyllium enrichment did not induce significantly different postprandial PYY response between the high- and low-fibre test products.
The enrichment of semisolid porridges with oat bran (3.5% DF), wheat bran (3.4% DF) or their combination (3.4% DF) did not significantly modify the postprandial ghrelin or PYY responses among the test products (Study II). Neither were there any significant differences in these responses between the DF levels, i.e. fibre-enriched vs. low-fibre porridges.
Viscosity. The decline in viscosity caused by depolymerization of beta-glucan in oat bran- enriched (3.4% DF) beverages affected the GI hormone responses (Study III). A beverage with enzymatically decreased viscosity induced greater postprandial decline in plasma ghrelin concentration than the one with higher viscosity (p=0.02). The test beverage with reduced viscosity induced also greater postprandial increase in the satiety-stimulating peptide, CCK, GLP-1 and PYY, concentrations and increased the total CCK, GLP-1 and PYY release (AUC) as compared with the beverage with higher viscosity (p≤0.05) (Figure 8).
Glucose and insulin responses
Fibre enrichment and fibre type. Postprandial glucose and insulin responses were affected by the use of soluble viscous DF in solid and semisolid food matrix. Both psyllium (10.0% DF) in solid vegetable patties (Study I) and oat bran (3.5% DF) in semisolid porridge (Study II)
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lowered postprandial glucose and insulin concentrations but with different intensity.
Psyllium enrichment suppressed glucose and insulin responses during the entire study period compared with the low-fibre products (p<0.05, (Figure 7 A, B). Also oat bran in the porridge produced the lowest overall plasma glucose response, especially at the end of the follow-up period (3 h), when compared with the porridges enriched with wheat bran or combination of oat and wheat bran or the one with no DF enrichment (Study II).
Postprandial insulin response was lower after oat bran enrichment already during the first hour of the follow-up period compared with the combination or wheat bran porridge (p<0.05) as well as at the end of the follow-up period (3h) compared with the wheat bran porridge. The enrichment of porridge with both oat and wheat bran elicited significantly increased postprandial insulin response at 45 (p=0.001) and 60 min (p=0.03) as compared with the meal with no DF enrichment.
Viscosity. Viscosity of the oat bran-enriched beverages in Study III modified also postprandial glucose and insulin responses (Figure 8 A, B). Low-viscous beverage increased postprandial glucose (p<0.05) and insulin responses (p<0.05) compared with the high-viscous beverage. The total postprandial insulin response (AUC) (p=0.007), but not the total glucose response, was higher after the beverages with lower viscosity.
Gastric function
Viscosity of the oat bran-enriched test products affected gastric emptying rate (Study III).
GE, as measured with the paracetamol absorption test, was enhanced after the low-viscous oat bran beverage compared with the high-viscous oat bran beverage. This was indicated as higher peak paracetamol concentration (at 90 min, p=0.020) and a tendency for a greater total paracetamol response (AUC) for the low-viscous beverage (p=0.051) (Figure 8 G).
Figure 7. Changes in the concentrations of A) plasma glucose, B) serum insulin, C) plasma ghrelin, D) plasma glucagon-like peptide 1 (GLP-1) and E) plasma peptide YY (PYY) during the 120 min postprandial period after the vegetable patties low or high in dietary fibre and/or protein in Study I. Corresponding areas under the curve (AUC) are presented as function of time. Values are means±SEM, n=16. WWB, white wheat bread; LFLP, low-fibre low-protein;
HFLP, high-fibre low protein; LFHP, low-fibre high-protein; HFHP, high-fibre high-protein.
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Figure 8. Changes in the concentrations of the plasma A) glucose, B) insulin, C) ghrelin, D) cholecystokinin (CCK), E) plasma glucagon-like peptide 1 (GLP-1), F) peptide YY (PYY) and G) serum paracetamol during the 180 min study period after the low- and high-viscous oat bran beverages in Study III. Values are means±SEM, n=20, except for CCK, n=17 and for paracetamol, n=10. For each time point, significant differences in means are indicated by asterisk (*); p<0.05, General Linear Model with Sidak adjustment.
5.1.2 Effects of dietary protein and its structure modification (Studies I, IV and V) Gastrointestinal hormone responses
Protein enrichment, protein type and protein structure modification. Protein enrichment, protein type (caseinate vs. whey protein) and crosslinking of caseinate in such a way that also the physical state of the test product changed from high-viscous to strong elastic gel (solid), affected the postprandial GI hormone release.
Soy protein enrichment together with psyllium fibre in vegetable patties (Study I) suppressed GLP-1 response for the entire experimental period, and the response was significantly lower compared with the low-fibre low-protein product (p<0.05) (Figure 7 D).
The crosslinked solid caseinate suppressed postprandial CCK response already in the beginning of the follow-up period (15 min) as compared with the test product including non-crosslinked caseinate (p<0.001). Crosslinked solid caseinate tended to suppress also the postprandial GLP-1 response compared with the non-crosslinked caseinate and liquid whey protein. On the other hand, the non-crosslinked caseinate stimulated CCK release more than the crosslinked solid caseinate or liquid whey protein (p<0.05). Neither the protein type nor the protein structure modification affected PYY response.
Glucose and insulin responses
Protein type and protein structure modification. Postprandial glucose and insulin responses were affected by protein enrichment (Study I) and by the modification of physical state and structure of the protein-rich test products (Study IV).
Even though psyllium enrichment of the vegetable patties (Study I) dominated glucose and insulin responses, also soy protein enrichment had effects. The soy-enriched high-fibre product lowered postprandial glucose response more than the low-protein high-fibre product (p<0.05). This was seen also as a smaller total glucose response (AUC) after high-protein versus the low-high-protein psyllium-enriched product (p<0.05). The total insulin response (AUC) was not affected by the soy protein enrichment.
The structure modification of caseinate affected glucose and insulin responses when the physical state of the test product was also changed (Studies IV). Both glucose and insulin responses were attenuated after the crosslinked solid caseinate compared with the non-crosslinked caseinate (p<0.05) (Study IV). However, when the structure modification changed only the molecular structure of the caseinate leaving the physical state unaffected, no significant differences in postprandial glucose and insulin responses were observed between the crosslinked and non-crosslinked caseinate beverages (Study V).
Postprandial glucose and insulin responses were not affected by the milk protein type (caseinate vs. whey protein) either when the physical state of the protein-rich test products clearly differed (Study IV) or when the physical state of the products was comparable (liquid) (Study V).
Gastric function
Protein type and protein structure modification. Gastric function, as assessed by GE rate, was not affected by crosslinking of caseinate, at least in the situation when the physical state of the products was comparable (Study V). Instead, the protein type seemed to influence GE;
GE (tlag) tended to be slower after the non-crosslinked caseinate than after the whey protein (p=0.061).
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Amino acid responses
Protein type and protein structure modification. As expected, whey protein- and caseinate-specific amino acid profile was reflected in individual postprandial amino acid concentrations (Study V). The concentrations of glutamic acid, lysine / tyrosine, isoleucine, leucine and tryptophan were increased more after whey protein than after non-crosslinked or crosslinked caseinate. On the other hand, postprandial concentrations of valine, histidine, phenylalanine and proline were lower after whey protein than after non-crosslinked or non-crosslinked caseinate. The milk protein-specific amino acid profile was also reflected in branched-chain amino acid and essential amino acid concentrations in which the levels were significantly higher after the whey protein than after the crosslinked and non-crosslinked caseinate (p<0.05). The postprandial total amino acid concentration was not significantly affected by the milk protein types.
Structure modification of caseinate, when the physical state was unaffected, did not significantly modify any of the individual amino acid responses when compared with the beverage with non-crosslinked caseinate (Study V). The postprandial branched-chain amino acid, essential amino acid and total amino acid responses were also comparable between the crosslinked and non-crosslinked caseinate (Study V).
5.2 EFFECTS OF DIETARY FIBRE AND PROTEIN ON APPETITE AND FOOD